The present invention relates to a fluorescent lamp that is operated with high frequencies in combination with an electronic ballast.
A large number of fluorescent lamps are turned on ordinarily with an electronic ballast, in which a capacitor is connected in parallel with a fluorescent lamp on the side opposed to a power source and in series with an electrode coil (hereinafter, this type of electronic ballast is referred to as a xe2x80x9cC preheat type electronic ballastxe2x80x9d). This is because a suitable electric current through a filament is required to preheat a fluorescent lamp cathode when it starts and to maintain the lighting, and a resonance voltage necessary for the lamp starting and operating should be ensured.
The reason this type of electronic ballast has spread most widely is that its circuit configuration is simple and inexpensive. In the C preheat type of electronic ballast, the current through a filament is relatively constant.
When a fluorescent lamp combined with the C preheat type of electronic ballast comes to the end of the life by the dissipation of the emissive coating on the electrode coil, the cathode fall voltage is raised. That results in the increase in the current through a filament, which causes the electrode coil to overheat by the excessive current. In addition to the heating from the electrode coil, an electrical discharge generates heat. Thus, the temperature in the vicinity of the electrode increases gradually. Under such circumstances, the lamp operation does not stop occasionally, even if the electrode coil is disconnected. In that case, the glass in the vicinity of the electrode between its terminals starts to be melted because of the constant-current property of the C preheat circuit, so that oscillation of the electronic ballast still continues after leakage of the fluorescent lamp.
In order to avoid these problems, the C preheat type of electronic ballast generally has the function of detecting a rise in the lamp voltage in accordance with a rise in the cathode fall voltage and cutting off an oscillation circuit beforehand or lowering an oscillation voltage to a safe level.
Furthermore, an electronic ballast in which another capacitor is added to the configuration of the above-described C preheat type of electronic ballast so as to be connected in parallel with a fluorescent lamp on the side nearer a power source (hereinafter, this type of electronic ballast is referred to as xe2x80x9cdouble C type electronic ballastxe2x80x9d) has been put to practical use before. This electronic ballast is doubted to be commercialized again in the future. For the double C type of electronic ballast, a large amount of oscillation voltage is always applied across the fluorescent lamp, even if the electrode coil is disconnected.
However, when the fluorescent lamp, which is combined with such a C preheat type of electronic ballast including a double C type for lighting, comes to the end of the life, the failure of detection of a rise in the lamp voltage, though it rarely occurs, may cause a bulb-end glass in the vicinity of the electrode, e.g., a stem glass to be melted, even if the electronic ballast has the function of detecting a rise in the lamp voltage and cutting off the oscillation circuit beforehand or lowering the oscillation voltage to a safe level. Thus, it has been demanded to solve these problems.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a fluorescent lamp in which a bulb-end glass is not melted after an electrode coil is disconnected in the last period of electrode life when the fluorescent lamp is turned on with a C preheat type electronic ballast, including a double C type.
A fluorescent lamp of the present invention includes a bulb provided with a pair of electrode coils at both ends thereof. Each of the electrode coils is mounted between two lead wires held by a bulb-end glass. A means for preventing overheating of the bulb-end glass is mounted between the lead wires located between the electrode coil and the bulb-end glass. The means for preventing overheating connects the lead wires electrically just before or after the electrode coil is disconnected.
This configuration can provide a fluorescent lamp that offers the excellent advantage of keeping the bulb-end glass safely at lower temperatures by electrically connecting the lead wires with the means for preventing overheating and of preventing the bulb-end glass from being melted, when an emissive coating is dissipated in the last period of electrode life of the fluorescent lamp, which ordinarily would increase the temperature of the electrode and its vicinities extraordinarily.
In a fluorescent lamp of the present invention, the means for preventing overheating has a first preferred configuration including a glass member and a first and a second metallic pin for supporting the glass member. One end of each of the first and the second metallic pin is connected to the lead wires, respectively. The first and the second metallic pin are provided not in contact with each other.
According to this preferred configuration, the glass member is heated by a conductive heat, a radiant heat, and intermittent pulse discharge after the emissive coating on the electrode coil in the last period of the life is dissipated and before the electrode is disconnected. In particular, the glass member in the base of the metallic pin is heated effectively by the intermittent pulse discharge. When the electrode coil is disconnected, ionic conduction occurs in the glass member, and thus the glass member starts melting. Furthermore, the two metallic pins may come into contact with each other by the flow of the molten glass member. This contact stops the glass member from melting (i.e., ionic conduction is interrupted). However, the electrical conduction (electronic conduction) between the metallic pins is continued.
Referring to another phenomenon, an increase in the current through a filament after emissive coating dissipation may cause the glass member to start melting because of the heat radiated from the electrode coil, even before the electrode coil is disconnected. In such a case, metal atoms sputtered from the electrode coil enter the molten portion of the glass member and bridge the two metallic pins, so that electronic conduction between the two metallic pins is established. Thus, a transition from the ionic conduction by the melting of the glass member to the electronic conduction occurs between a pair of metallic pins, and thereby the electrical conduction can be continued.
During the above period, the bulb-end glass is not melted, so that the fluorescent lamp can be protected against an excessive heat and maintained safely. Furthermore, even if the lamp in the above condition is restarted after it is turned off, the bulb-end glass is not melted. Thus, the fluorescent lamp can be maintained safely.
According to the first preferred configuration, since the glass member is held by a pair of metallic pins at both ends thereof and each of the metallic pins is connected to the two lead wires, respectively, the glass member can be mounted easily between the lead wires.
In the first configuration, the means for preventing overheating further may include a metallic container in which the glass member is housed. At least one of the first and the second metallic pin supports the glass member indirectly by supporting the metallic container. The glass member is housed in the metallic container so that a portion of the glass member is exposed to a discharge space.
According to this configuration, when the electrode coil in the last period of the life, in which an emissive coating has been dissipated, is disconnected, the glass member starts melting and conducting ionically. However, since the glass member is housed in the metallic container, the molten state can be maintained in the metallic container without producing a significant deformation of the glass member. During this period, the bulb-end glass is not melted, so that the fluorescent lamp can be maintained safely.
In the above configuration, it is preferable that the portion of the glass member exposed to the discharge space faces to the electrode coil. According to this preferred configuration, the portion of the glass member exposed to the discharge space can be locally heated effectively by the heat radiated from the electrode coil or the intermittent pulse discharge from the opposite electrode. This can ensure that the glass member is melted faster than the bulb-end glass.
Furthermore, it is preferable that one of the metallic pins is inserted into the glass member and the other is connected to the metallic container in which the glass member is housed. This preferred configuration allows the shape of the molten glass member to be maintained in the metallic container. In addition, a set of mounted members (means for preventing overheating) thus formed can be manufactured at a low price.
Furthermore, it is preferable that one of the metallic pins, which has been inserted into the glass member, has a fastener, and that the fastener comes into contact with the end surface of the glass member. Also, the length of the glass member housed in the metallic container in the insertion direction of the metallic pin is longer than the distance from the bottom face of the metallic container to the top in the insertion direction of the metallic pin. According to this preferred configuration, the glass member is fixed between the fastener of one of the metallic pins and the metallic container, and thus it does not fall off in any orientations of the lamp during operation. In addition, since the length of the glass member is longer than the depth of the metallic container, a portion of the glass member is projected from the metallic container and exposed directly to the source of radiant heat or a discharge space. As a result, the exposed portion of the glass member can be heated effectively by a conductive heat, a radiant heat, and intermittent pulse discharge after the emissive coating on the electrode coil in the last period of the life is dissipated and before the electrode is disconnected. After the disconnection of the electrode coil, the exposed portion of the glass member can be melted faster than the bulb-end glass. Furthermore, the molten glass member can be maintained at the position where it has been melted (in the metallic container) by the metallic pin having the fastener and the metallic container.
It is preferable that the end of the opening of the metallic container, in which the glass member is housed, is bent inward. According to this preferred configuration, the glass member does not fall off the metallic container before it is melted, regardless of the orientation of the lamp during operation. In addition, after the glass member is melted, the welding surface of the glass member adheres to the inner surface of the metallic container, which can prevent the glass member from falling off the metallic container.
It is preferable that the metallic container in which the glass member is housed is held by the metallic pins via an electrical insulator, and that a pair of metallic pins are provided in close proximity in the glass member. According to this preferred configuration, by adjusting the distance between a pair of metallic pins that are insulated electrically from the metallic container, the impedance between the lead wires in the glass member can be determined easily so as to ensure that the glass member in the metallic container is melted when the electrode coil is disconnected. In addition, this configuration can prevent the molten glass member from flowing out of the metallic container.
It is preferable that the surface of the glass member in the first configuration of the means for preventing overheating is coated with a non-conductive inorganic heat-resisting material.
According to this preferred configuration, the glass member is heated by a conductive heat, a radiant heat, and intermittent pulse discharge after the emissive coating on the electrode coil in the last period of the life is dissipated and before the electrode is disconnected. When the electrode coil is disconnected, the glass member starts melting and conducting ionically. However, since the outer surface of the glass member is coated with an inorganic heat-resisting material, the molten state can be maintained without producing a significant deformation of the glass member. During this period, the bulb-end glass is not melted, so that the fluorescent lamp can be maintained safely.
In the above configuration, it is preferable that both metallic pins are inserted into the glass member, and that the distance between the metallic pins is substantially equal to or shorter than the insertion length of the metallic pin into the glass member. This preferred configuration can prevent the molten glass member from falling off the metallic pins. In addition, the shape of the glass member can be maintained without being cut off by melting.
It is preferable that the point of the metallic pin in the glass member. differs from a portion that continues on to the point in cross section, or has a thickness larger than that of the portion that continues on to the point. This preferred configuration reliably can prevent the molten glass member from falling off the metallic pins.
It is preferable that the inorganic heat-resisting material has a melting point in excess of 200xc2x0 C. or more above a softening point of the glass member. According to this preferred configuration, the inorganic heat-resisting material is not deformed, even at temperatures at which the glass member is melted. Thus, the glass member coated with the inorganic heat-resisting material is not cut off by melting, so that the shape of the glass member can be maintained substantially against the effect of gravity when a lamp is turned on.
It is preferable that a substance having a lower work function, more preferably cesium oxide, is attached to the surface of the metallic pin. This preferred configuration allows ion bombardment heating caused by main discharge between the electrodes to be concentrated on the metallic pins having a lower work function on the surface. Thus, the glass member rather than the bulb-end glass can be melted certainly.
Next, in a fluorescent lamp of the present invention, the means for preventing overheating has a second preferred configuration including a glass member mounted between the lead wires and a means for preventing falling of the glass member from the lead wires during melting.
According to this preferred configuration, the glass member is heated by a conductive heat, a radiant heat, and intermittent pulse discharge after the emissive coating on the electrode coil in the last period of the life is dissipated and before the electrode is disconnected. When the electrode coil is disconnected, the glass member starts melting and conducting ionically. However, the glass member does not fall off the lead wires because of the means for preventing falling, and thus the molten state can be maintained. During this period, the bulb-end glass is not melted, so that the fluorescent lamp can be maintained safely.
In the above configuration, the means for preventing falling can be provided on the circumference of the glass member. Furthermore, the means for preventing falling can be formed of a non-conductive inorganic heat-resisting material (e.g., ceramic coating) or a metallic band. This configuration can facilitate manufacturing of the means for preventing overheating provided with the means for preventing falling.
Next, in a fluorescent lamp of the present invention, it is preferable that the means for preventing overheating has a third preferred configuration including a glass member, and that an electrical volume resistance of the glass member is lower than that of the bulb-end glass. According to this preferred configuration, when the electrode coil is disconnected, the glass member rather than the bulb-end glass is melted and ionically conducted selectively. Thus, the bulb-end glass is not melted, so that the fluorescent lamp can be maintained safely.
Furthermore, in a fluorescent lamp of the present invention, it is preferable that the means for preventing overheating has a fourth preferred configuration including a glass member, and that the electrical conduction between the lead wires through the glass member is continued just before or after the electrode coil is disconnected. According to this preferred configuration, the glass member has been heated by a conductive heat, a radiant heat, and intermittent pulse discharge after the emissive coating on the electrode coil in the last period of the life is dissipated and before the electrode is disconnected. The glass member becomes conductive ionically and is melted selectively before or after the electrode coil is disconnected. Thus, the bulb-end glass is not melted, so that the fluorescent lamp can be maintained safely.
In a fluorescent lamp of the present invention, it is preferable that at least a portion of the surface of the bulb-end glass in the lamp is coated with a non-conductive inorganic heat-resisting material. According to this preferred configuration, the bulb-end glass supporting the lead wires is not heated locally by ion bombardment caused by main discharge between the electrodes. Thus, the glass member in the means for preventing overheating can be melted certainly faster than the bulb-end glass.
In a fluorescent lamp of the present invention, it is preferable that the means for preventing overheating is located closer to the electrode coil than to the bulb-end glass. This preferred configuration allows the means for preventing overheating to be subjected more to the heat radiated from the electrode coil that glows red-hot before disconnection. Thus, when the electrode coil is disconnected, the glass member in the means for preventing overheating can be melted faster than the bulb-end glass.